Surfaces with extreme wetting properties are of special interest and practical utility in many commercial, industrial, military, and biotechnological contexts. Synthetic materials that are completely non-wetting to water (superhydrophobic), to oils (superoleophobic), or to both oil and water (superomniphobic) have enabled the design of self-cleaning and anti-fouling surfaces, new generations of smart textiles and stain-resistant clothing, and innovative methods for the collection and manipulation of complex fluids, including approaches to oil recovery and oil/water separation (see, for example, Genzer et al., Science (2000), 290: 2130; Liu et al., Chem. Soc. Rev. (2010), 39:3240; Tuteja et al., Science (2007), 318:1618; Bellanger et al., Chem. Rev. (2014), 114: 2694; Deng et al., Science (2012), 335:67; Chu et al., Chem. Soc. Rev. (2014), 43: 2784; Yuan et al., Nat. Nanotechnol. (2008), 3:332; Deng et al., Adv. Mater. (2010), 22: 5473; Yao et al., Adv. Mater. (2011), 23:719; Jin et al., Adv. Mater. (2011), 23: 2861; Banerjee et al., Adv. Mater. (2011) 23: 690; Kota et al., Nat. Commun. (2012), 3:1; Ueda et al., Adv. Mater. (2013), 25: 1234; Timonen et al., Science (2013), 341:253; Tian et al., Adv. Mater. (2014), 26: 6872).
Advances toward these and other emerging applications have been made possible by an understanding of the ways that features found on natural non-wetting surfaces (e.g., the lotus leaf) (see Barthlott et al., Planta (1997), 202:1) work in synergy to promote anti-fouling behavior, (Gao et al., Nature (2004), 432:36) and by the development of new approaches to the fabrication of hard and soft material interfaces (Bae et al., Adv. Mater. (2014), 26:675) that can recapitulate those critical features on synthetic surfaces better suited for everyday use (so-called “bio-inspired” approaches to materials design).
One major challenge to the application of synthetic non-wetting surfaces in practical settings lies in developing materials that are durable and able to withstand the rigors of use without loss of special wetting behavior (Deng et al., Science (2012), 335:67; Li et al., Angew. Chem. Int. Ed. (2010), 49:6129; Deng et al., Adv. Mater. (2011), 23:2962; and Manna et al., Adv. Mater. (2013), 25:5104). The susceptibility of many synthetic non-wetting surfaces to physical insults, for example, is commonly regarded as an “Achilles heel” with respect to practical utility (Verho et al., Adv. Mater. (2011), 23:673; lonov et al., Phys. Chem. Chem. Phys. (2012), 14:10497).
Other key challenges lie in developing anti-fouling interfaces that remain functional in harsh and chemically complex media—e.g., at extremes of pH and ionic strength, or upon contact with surface-active agents that can also adsorb and compromise non-wetting behavior (Deng et al., Adv. Mater. (2010), 22:5473). Non-wetting surfaces that permit interfacial properties to be tuned or spatially patterned could also open the door to new and advanced applications of these “super-phobic” materials. (Ueda et al., Adv. Mater. (2013), 25:1234; and Parker et al., Nature (2001), 414:33).
Recent reports describe materials that exhibit “underwater superoleophobicity”, or surfaces that are extremely non-wetting to oils and organic liquids only when submerged in water (Tian et al., Adv. Mater. (2014), 26: 6872; Liu et al., Adv. Mater. (2009), 21:665; Jung et al., Langmuir (2009), 25:14165; Liu et al., Adv. Mater. (2012), 24:3401; Nishimoto et al., RSC Adv. (2013), 3:671; and Cai et al., Adv. Funct. Mater. (2014), 24:809). This unique behavior contrasts to that of conventional superoleophobic materials, on which organic liquids “bead up” and “roll off” when brought into contact under air (Tuteja et al., Science (2007), 318:1618) (superoleophobicity is defined here by an advancing oil contact angle (θ)≥150° and a roll-off angle ≤10°).
Synthetic surfaces that exhibit underwater superoleophobicity have emerged only recently, based on designs that mimic, to varying extents, critical physical and chemical features found on the scales of fish (Liu et al., Adv. Mater. (2009), 21: 665; and Cai et al., Adv. Funct. Mater. (2014), 24:809) and other aquatic anti-oil-fouling surfaces (Liu et al., Adv. Mater. (2012), 24:3401; and Nishimoto et al., RSC Adv. (2013), 3:671). These materials typically have two key elements in common: (i) rough surfaces that present micro- and nanoscale topographic features, and (ii) an ability to adsorb or host water near their surfaces to minimize contact with oily liquids creating a conceptual and theoretical framework similar in principle to superhydrophobic surfaces with multiscale features that can “trap” air to repel water (Tian et al., Adv. Mater. (2014), 26:6872; and Liu et al., Adv. Mater. (2009), 21:665). Previous studies have used hydrogels, polyelectrolyte assemblies, and metal oxide nanorods to create model surfaces meeting these criteria (see, for example, Liu et al., Adv. Mater. (2009), 21:665; Lin et al., Adv. Mater. (2010), 22:4826; Xu et al., Adv. Mater. (2013), 25:606; Xu et al., ACS Nano (2013), 7:5077; Ma et al., Adv. Mater. Interfaces (2014), 1:1300092; Liu et al., ACS Nano (2012), 6:5614; and Zhang et al., Adv. Mater. (2013), 25: 2071).
Those approaches have advanced an understanding of key principles underlying underwater superoleophobicity, but they lead to soft surfaces and coatings that are susceptible to physical damage or disruption, particularly in harsh and chemically complex environments. They also provide limited means to tune non-wetting behavior or define and vary other important physicochemical or interfacial properties.